I never said that being a PoH gave her offensive capabilities, I only meant that as a character being both is a bit of overkill i.e. as a character it sort of makes her overpowering.
That's true only if they actually do something with both aspects. The PoH aspect as a whole was greatly mistreated, and if Kairi's to maintain being an active plot force in the series, best to not have it be only as Sora's would be romantic interest as that only worked really well when we had Namine and Xion's own plots to go along with it. If they want to use Kairi herself, however, they need more than that.
While in terms of power she appears to be ridiculously strong with the addition of the Keyblade
Against intimidating
SHADOW HEARTLESS.
I'll never get over this. I'm not saying she can't become stronger but just because she smacked a couple of SHADOW Heartless with what's the strongest weapon known to man doesn't make her strong; it makes her capable of swinging a blunt object.
Also in reading the Ansem reports it mentions that he opened the door in Radiant Garden. With Ansem the Wise not being a wielder I thought that this meant that others could open unlocked doors, though looking back on it I'm not sure whether it was Ansem the Wise or Xehanort the Apprentice who opened the door. I just know that one of the reports mentions it just before he meets the king.
It's Apprentice Xehanort, not AtW. AtW himself wrote in the KH2 Secret Reports that Xehanort opening the door was one of the biggest mysteries he came face to face with.
Think about it this way - had non-wielders been capable of opening doors, I think we'd have heard of a lot more instances of it happening rather than just Xehanort and Riku - one with basically Wielding status shoved down our throat long before you even glance at any BBS material, and Riku, the originally chosen Wielder for the KK who had the KK (albeit unchosen by it) when he opened the door to DI.
On another point, they knew that the doors led to the Hearts of the Worlds and wanted to attain that power. Therefore needing to open the door to gain it. At least that's how I interpreted the first game and they're intentions. In any case, they did mention that Sora could be useful if they could manage to manipulate him as they were Riku. Couldn't that mean they intended to use one or the other to open the doors?
The Heartless could unlock the doors as well. Therefore, even while lacking Keyblades on "their side", numerous worlds were overrun by the Heartless and vanished. Riku opening the door was what summoned them, or so it seemed, but wasn't necessary for it.
As for using either Riku or Sora, seeing how the KK was originally Riku's and Maleficent did play on his jealousy issues, I'd sooner say they were trying to get Riku to snatch the Keyblade back than use Sora, as he seemed a lot more manipulatable, and a lot more reachable.